Investment casting by the lost wax process can be traced to ancient Egypt and China. The process as practiced today, however, is a relatively new technology dating to the 1930's and represents a rapidly growing business and science. Investment casting technology simplifies production of complex metal shapes by casting molten metal into expendable ceramic shell molds formed around disposable wax preforms which duplicate the desired metal shape. “Precision Investment Casting”, i.e., PIC, is the term in the art that refers to this technology.
The conventional PIC process employs six major steps:
1. Preform Preparation:
A disposable positive preform of the desired metal casting is made from a thermoplastic material such as wax that will melt, vaporize or burn completely so as not to leave contaminating residues in the de-waxed ceramic shell mold. The positive preform is prepared by injecting the thermoplastic material into a negative, segmented, metal die or “tool” designed to produce preforms of the shape, dimension and surface finish required for the metal casting. Single or multiple preforms can be assembled by fusing them to a disposable wax “sprue system” that feeds molten metal to fill the shell mold;
2. Shell Mold Construction by:
(a) dipping the preform assembly into a refractory slurry having fine particulate refractory grain in an aqueous solution of alkali stabilized colloidal silica binder to define a coating of refractory material on the preform;
(b) contacting the refractory coating with coarse dry particulate refractory grain or “stucco” to define a stucco coating, and
(c) air drying to define a green air dried insoluble bonded coating. These process steps can be repeated to build by successive coats a “green”, air dried shell mold of the desired thickness.
3. Dewaxing—The disposable wax preform is removed from the “green” air dried shell mold by steam autoclaving, plunging the green shell mold into a flash de-waxing furnace heated to 1000° F.–1900° F., or by any other method which rapidly heats and liquefies the wax so that excessive pressure build-up does not crack the shell mold.
4. Furnacing—The de-waxed shell mold is heated at about 1600° F.–2000° F. to remove volatile residues and form stable ceramic bonds in the shell mold.
5. Pouring—The heated shell mold is recovered from the furnace and positioned to receive molten metal. The metal may be melted by gas, indirect arc, or induction heating. The molten metal may be cast in air or in a vacuum chamber. The molten metal may be poured statically or centrifugally, and from a ladle or a direct melting crucible. The molten metal is cooled to produce a solidified metal casting in the mold.
6. Casting recovery—The shell molds having solidified metal castings therein are broken apart and the metal castings are separated from the ceramic shell material. The castings can be separated from the sprue system by sawing or cutting with abrasive disks. The castings can be cleaned by tumbling, shot or grit blasting.
Investment casting shell molds tend to be fragile and prone to breakage. In an effort to improve the strength of investment casting shell molds, small amounts of chopped refractory fibers and or in combination with chopped organic fibers have been added to aqueous refractory slurries. Refractory slurries which include these small amounts of chopped refractory fibers enable application of thicker coatings to a preform. These slurries, however, require significant additions of polymer to achieve satisfactory green strength and flow properties of the slurry.
A need therefore exists for materials and methods which provide investment casting shell molds which have improved strength and avoids the disadvantages of the prior art.